Postcontractile motoneuronal discharge produced by muscle afferent activation.

Muscle afferent activity is markedly enhanced following isometric contractions in hindlimb musculature of various vertebrates. Such increased and persistent postcontractile sensory discharge (PCSD) consists for the most part of potentiated firing of muscle spindle primary afferents. To determine if a positive feedback function is served, involving recruitment of homonymous and synergistic motor units, 62 motoneurons were isolated in L7 ventral root filaments, categorized according to their reflex response to triceps surae stretch, and monitored before and after isometric contraction for variations in spike frequency. Motoneurons found reflexly coupled to triceps surae stretch receptors significantly increased (P less than .01) their firing frequency during PCSD. Tonic motoneurons, believed to be primarily of alpha origin, accounted for most of the observations. This latter finding is consistent with known effects of spindle afferent input on motoneuronal recruitment. Functionally, this autogenetic control circuit should enhance "muscle stifness" and stabilize the muscle against minor perturbations.